1. Field of the Invention
This invention relates to a signal generator, and more particularly to a signal generator which can be most suitably used for a writing portion to a memory in a time base corrector in a PAL color television system.
2. Description of the Prior Art
FIG. 1 shows a conventional writing portion to a memory in a time base corrector. In FIG. 1, video signals are supplied to an AD converter 1, and converted thereby to digital signals. The converted video signals are memorized in a memory 2. In this case, the memory 2 may be of the type of RAM (Random Access Memory).
In order to obtain a writing address signal for the memory 2, a burst signal is supplied to a locked oscillator 3. A signal S synchronized with the burst signal, whose frequency is fc, is obtained from the locked oscillator 3, and applied to a counter 4 to be counted by the latter. The writing address signal is obtained from the counter 4. The output signal S from the locked oscillator 3 is further supplied as a clock pulse to the AD converter 1.
In the read-out of the video signals memorized in the memory 2, a reading address signal is supplied to the memory 2 from a not-shown circuit. The time base of the video signals is corrected with the adjustment of the time relationship between the writing address signal and the reading address signal.
In the time base corrector of FIG. 1, pulse signals H synchronized with horizontal synchronizing signals are supplied as reset signals to the counter 4. Accordingly, when the time base corrector of FIG. 1 is used for a PAL color television system, it has the following disadvantage:
According to the PAL color television system, there is the following relationship between the frequency fc of the subcarrier (fc=4.43361875 MHz) and the frequency f.sub.H of the horizontal synchronizing signal (f.sub.H =15.625 KHz): EQU fc=(284-1/4)f.sub.H +25 Hz (1).
Accordingly, the address signal obtained by resetting the counter 4 with the pulse signal H synchronized with the horizontal synchronizing signal progresses or leads in phase by about .pi./2 with respect to the output signal S of the locked oscillator 3 every period of the horizontal scanning line. When the video signals written into the memory 2 with the thus obtained address signals are read out from the memory 2 with address signals obtained by dividing a fixed frequency, the video signals are distorted. For example, when video signals for a vertical line as a picture are written into the memory 2, the read-out video signals are distorted into video signals for zigzag configuration.